A known machine for producing helically wound lock-seam tubing is disclosed in British patent publication GB 2,213,748 (see FIG. 1) which is incorporated herein by reference. In this machine, a metal strip is fed to a forming head by means of drive rollers, and a helically wound lock-seam tube is formed by clinching rollers associated with the forming head.
Another known machine for producing helically wound tubing is disclosed in British patent specification GB 1,168,178, wherein FIGS. 3 and 4 show different seams.
The technology related to such machines is well-known to persons skilled in the art. At present, several further developed machines of a similar type are available on the market. However, these machines still suffer from some specific drawbacks.
In the production of helically wound lock-seam tubes of the present type, the lock-seam tends to slip, which results in an undesirable change in tube diameter. The general solution to this problem is to increase the pressure of the clinching rollers acting on the lock-seam in the forming head. However, such an increased clinching pressure can cause damage to and deficiencies in the lock-seam as well as in the adjacent portions of the metal strip forming the tube. Thus, the adjustment of the clinching pressure is crucial to the quality of the lock-seam and the resulting tube.
In addition to the clinching pressure, the operator running the machine must consider several other parameters, such as the thickness of the metal strip, the lubrication of the same, and possible wear of different machine components, for example the rollers. Thus, great demands are made on the skill and experience of the operator.
In spite of the operator's skill, it is hard to avoid slip or sliding in the lock-seam. An undesirable increase of the tube diameter can occur either in production, immediately after the forming head, or during storing, handling and transportation of the tubes. It should be noted that the tube diameter may also be undesirably reduced due to lock-seam slip, for example when tubes are subject to pressure forces from the outside.
The change in tube diameter is always a problem, since the tubes delivered do not fulfil the customers' specifications and requirements.
Specific problems are encountered when using the tubes in ventilation duct systems mounted on different premises. For example, vibrations caused by fans and similar equipment can initiate lock-seam slip which, in turn, leads to undesirable leakage of air and pressure drops. In severe cases, lock-seam slip of the ventilation ducts can jeopardize safe function of the whole ventilation system.
In ventilation duct systems, different fittings are connected to the helically wound lock-seam tubes forming the major part of the system. Such fittings are bends, T-pieces, dampers, sound attenuators, etc. The fittings are normally inserted in the end of the tubes, and a sealing ring on the inserted portion of the fitting ensures safe sealing. However, if the tube has an undesirably increased diameter, there is a risk that sufficient sealing cannot be obtained in the joint between the fitting and the tube. This may lead to the leakage and pressure drop problems discussed above, resulting in energy losses and increased running costs.
In other cases, the fitting is not provided with a pre-mounted seal, but the joint between the outside of the fitting and the inside of the tube is filled with a mastic for sealing purposes. If the gap is too large due to an undesirable increased tube diameter, sufficient sealing may not be obtained by means of the mastic. Even if the mastic is safely fastened, problems may arise later due to increased tube diameter caused by vibrations, air pressure peaks, etc.
Further, change in tube diameter is disadvantageous when prefabricated annular fastening devices and the like are to be applied on the outside of the tube. Such devices do not fit if the tube diameter has changed.